linux-kernel - Re: [patch 4/5] PTP: add PTP_SYS_OFFSET emulation via cross timestamps infrastructure

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Date:   Fri, 20 Jan 2017 15:02:16 +0100
From:   Radim Krcmar <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Richard Cochran <richardcochran@...il.com>,
        Miroslav Lichvar <mlichvar@...hat.com>
Subject: Re: [patch 4/5] PTP: add PTP_SYS_OFFSET emulation via cross
 timestamps infrastructure

2017-01-20 14:36+0100, Paolo Bonzini:
> On 20/01/2017 14:07, Marcelo Tosatti wrote:
>> On Fri, Jan 20, 2017 at 01:55:27PM +0100, Paolo Bonzini wrote:
>>>
>>>
>>> On 20/01/2017 13:20, Marcelo Tosatti wrote:
>>>>  kernel/time/timekeeping.c        |   79 +++++++++++++++++++++++++++++++++++++++
>>>
>>> Why not leave this in drivers/ptp/ptp_chardev.c?
>> 
>> timekeeper_lock
> 
> Why does emulate_ptp_sys_offset need it, if the current PTP_SYS_OFFSET
> code doesn't?  Is the latency acceptable (considering this is a raw spin
> lock) or is there a seqlock that we can use instead (such as tk_core.seq
> like in get_device_system_crosststamp)?

The spinlock prevents writers to take the tk_core.seq, which means that
time cannot be changed during that.

The simplest alternative would be to use tk_core.seq for all our reads,
but that would increse the chance of re-reading, even infinitely.

But we don't need to read everything with the same time base -- if the
time is changed (by NTP/user/...) between our reads, then the value will
be off, but if writer took tk_core.seq just to accumulate current time,
then the time after accumulation stays the same and it would work as if
we had the tk_core.seq for the whole time.

Another solution would be to do just one one read and set it to all
saples -- the difference between t[i] and t[i+2] would be 0.  We are
quite sure the just one read is enough, this hack could be even better.

>>>> +		if (ptp->info->emulate_ptp_sys_offset_mean) {
>>>> +			err = emulate_ptp_sys_offset(ptp->info, sysoff, arg);
>>>> +			break;
>>>> +		}
>>>
>>> I think this should be simply "if (!ptp->info->gettime64)" and,
>>> likewise, there should be an emulation based getcrosststamp in
>>> ptp_clock_gettime.
>>>
>>> Paolo
>> 
>> gettime64 is called directly via ptp_clock_gettime.
> 
> Yes, but ptp_clock_gettime can be taught to use getcrosststamp instead.

I agree,

  if (!gettime64)
      use getcrosststamp

and KVM PTP device will not implement gettime64().